H2—O2(air) fuel cells are well known in the art and have been proposed as a power source for many applications. There are several types of H2—O2 fuel cells including acid-type, alkaline-type, molten-carbonate-type, and solid-oxide-type. So called PEM (proton exchange membrane) fuel cells (a.k.a. SPE (solid polymer electrolyte) fuel cells) are of the acid-type, potentially have high power and low weight, and accordingly are desirable for mobile applications (e.g., electric vehicles). PEM fuel cells are well known in the art, and include a “membrane electrode assembly” (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack.
In PEM fuel cells hydrogen is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can either be in a pure form (i.e., O2), or air (i.e., O2 mixed with N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprise finely divided catalytic particles (often supported on carbon particles) admixed with proton conductive resin.
When air is used as the cathode reactant, nitrogen within the cathode side of the fuel cell stack permeates into the anode side of the fuel cell stack across the membrane separating the anode and cathode flow fields. The nitrogen interferes with reaction of the hydrogen by decreasing the hydrogen partial pressure. As the nitrogen concentration increases in the anode side of the fuel cell stack, voltage production of the fuel cell stack will decrease. The decrease in voltage production can be localized to specific fuel cells or can exist throughout all the fuel cells. If the nitrogen concentration gets to high, the fuel cell stack can become fouled by the nitrogen thereby starving the reaction and resulting in unstable voltage production. It is therefore advantageous to prevent nitrogen fouling of the fuel cell stack.
To prevent nitrogen fouling of the fuel cell stack, the nitrogen can be vented from the anode side along with anode effluent, which contains unused hydrogen. The venting of unused hydrogen, however, reduces the efficiency of the fuel cell stack and limits the operating range of the fuel cell stack for a given quantity of stored hydrogen. Thus, the need for removing nitrogen from the anode side and the desire to avoid venting unused hydrogen must be balanced. Accordingly, it would be advantageous to provide a control scheme wherein the necessity of venting nitrogen from the anode side is balanced against the desire to provide efficient operation of the fuel cell stack and an acceptable operating range. It would further be advantageous if such a control scheme avoided the use of costly dedicated sensors or similar devices to determine and/or monitor the nitrogen concentration in the anode side. Moreover, it would be advantageous if such a control scheme utilized existing hardware present in the fuel cell system.